12

This is question is inspired by the question: In what areas does F# make "absolute no sense in using"?

In theory, it should be possible to use any .NET supported language in a single project. Since every thing should be compiled into IL code, then linked into a single assembly.

Some benefits would include the ability to use say F# for one class, where F# is more suited to implement it's function, and C# for another.

Is there some technical limitation I'm overlooking that prevents this sort of setup?

Community
  • 1
  • 1
CodeNaked
  • 40,753
  • 6
  • 122
  • 148

8 Answers8

13

A project is restricted to a single language because, under the hood, a project is not much more than an MSBuild script which calls one of the command-line compilers to produce an assembly from the various source code files "contained" in the project folder. There is a different compiler for each language (CSC.exe is for example the one for C#), and what each project has to do to turn its "contained" source code into an assembly differs with each language.

To allow multiple languages to be compiled into a single assembly, the project would basically have to produce an assembly for each language, then IL-Merge them. This is costly, requires complex automation and project file code generation, and in most circumstances it's a pretty fringe need, so the VS team simply didn't build it in.

KeithS
  • 70,210
  • 21
  • 112
  • 164
  • 15
    I think producing assembly for each language and IL-merging them is not really the problem. The problem is that you'd need to deal with circular references between the two parts of the project. This essentially means that you cannot easily compile any of the two parts (which came first, an egg or a chicken?) – Tomas Petricek Mar 25 '11 at 15:11
  • 1
    @Tomas - That could be solved by enforcing the ordering of files, like F# does. – Robert Jeppesen Mar 25 '11 at 15:22
  • @Robert: Yes, it could be. But what would be the advantages of having that instead of simply having separate libraries? (And then ILmeging them in a post-build task) – Tomas Petricek Mar 25 '11 at 15:50
  • 2
    @Tomas - I realize this is never going to happen, of course, but it would lower the barrier of entry for people wanting to dip their toes in F#. :) For old projects I would love to be able to just have a couple of F# files in there without having to refactor the entire code base to suit this requirement. – Robert Jeppesen Mar 25 '11 at 16:06
  • @Robert: Yes, I agree it would be quite useful. The problem is that enforcing the ordering of files in a combined C# / F# project would mean that you'd have to refactor the C# code base quite a bit too. (To avoid circular references). – Tomas Petricek Mar 25 '11 at 17:13
  • I think a reasonable option is to define some interface in C# (e.g. `ICalculationEngine`), referencing C# project from F# & implementing the interface (you have access to all C# types). Then the C# project just needs to dynamically load the F# implementation and use it. – Tomas Petricek Mar 25 '11 at 17:15
9

While projects are restricted to a single language, a solution is not... and solutions can contain multiple projects.

Powerlord
  • 87,612
  • 17
  • 125
  • 175
  • BUT using only the IDE it's impossible (I think) to have a single .EXE from multiple projects. – xanatos Mar 25 '11 at 14:56
  • 3
    One of the projects would be the EXE, and another would be a DLL that sits alongside and is called by the EXE. – KeithS Mar 25 '11 at 14:59
  • 3
    Or one project can produce a .netmodule and the other project can consume that .netmodule into its assembly resulting in a single .exe. – ildjarn Mar 25 '11 at 15:44
7

As others mentioned, a project is a stand-alone unit that is compiled by a single compiler.

I hear the question about including e.g. one F# type in a larger C# project quite often, so I'll add some details from the F# specific point of view. There are quite a few technical problems that would make it really difficult to mix F# and C# in one project:

  • C# compiler sees all classes, while F# type declarations are order-dependent. I'm not sure how would you even specify what types should the F# code see at which point.
  • F# compiler needs to know how declarations are used in order to infer types. Would it also get usage information from the C# compiler?
  • The two compilers use different representation of types. When compiling, there are no System.Type information, so how would they share the information? (Both of them would need to agree on some common interface that allows them to include language-specific information - and the information may be also incomplete).

I think this is enough to explain that doing this is not just a feature that may or may not be done depending on the schedule. It is actually an interesting research problem.

Tomas Petricek
  • 240,744
  • 19
  • 378
  • 553
5

For what it's worth, it's possible to have ASP.NET projects that use C# and VB.NET (or anything else, you define the compilers in web.config), just in different files.

Blindy
  • 65,249
  • 10
  • 91
  • 131
3

All code files are processed by a single compiler. That's why a project can only contain a single language.

Mixing languages may not make much sense either, since each language generates it's own IL.

This of course doesn't restrict you form having multiple projects from different langauges in the same solution, since each project is compiled independently

Carlos Muñoz
  • 17,397
  • 7
  • 55
  • 80
2

Consider using ILMerge if you want to maintain a single .exe or .dll built by a number of different compilers.

SK-logic
  • 9,605
  • 1
  • 23
  • 35
2

Technically, you can mix languages in a single project, if one (or more) of those languages are scripting languages. See How to use Microsoft.Scripting.Hosting? for more details.

I know this isn't what you were talking about, but it's a little fun fact if you weren't aware.

Community
  • 1
  • 1
Giovanni Galbo
  • 12,963
  • 13
  • 59
  • 78
2

The project file is nothing but an elevated list of command line parameters to the relevant compiler. A file with the extension of .csproj contains the parameters for a C# compiler, .vbproj for the VB.NET compiler and so on.

You can however create two or more projects in the same solution file, one for each language and then link them together in one exe file using ILMerge.

steinar
  • 9,383
  • 1
  • 23
  • 37